In superchargers, a thrust force is generated in a rotary shaft due to a pressure difference on a compressor side and a turbine side. Therefore, a thrust bearing is provided to stop the movement of the rotary shaft in a thrust direction. As the thrust bearing, bearings with simple structure, such as a taper-land type thrust bearing, are often used. However, when the rotating speed of the rotary shaft is fast, a centrifugal force acts on lubricating oil outward in the radial direction of the rotary shaft. Therefore, the outflow of the lubricating oil to an outer peripheral side increases.
A thrust bearing is arranged to face the rotary shaft of a supercharger, and thrust collars that apply a thrust force to the thrust bearing are fixed to the rotary shaft. Since it is generally difficult to make the thrust collars parallel to the thrust bearing without an error, a gap may be increased in outer peripheral regions of facing surfaces between the thrust bearing and the thrust collars. When the rotating speed of the rotary shaft is fast and a large centrifugal force acts on the lubricating oil, if there is an increased gap on outer peripheral sides of the facing surface, there is a concern that a large amount of lubricating oil may flow out from this gap, and the bearing capability of the thrust bearing may decline extremely.
A thrust bearing device for an exhaust gas turbo supercharger is disclosed in PTL 1. This thrust bearing device includes a turbine wheel and a compressor wheel that are fixed to a rotary shaft, a turbine-side thrust collar and a compressor-side thrust collar that are fixed to the rotary shaft, and a thrust bearing that is fixed to a bearing housing, has the two thrust collars coming into sliding contact therewith from both sides, and receives a thrust load applied to the rotary shaft.
A taper-land type thrust bearing provided in fluid machinery, such as a water wheel and a pump, is disclosed in PTL 2. PTL 2 suggests the thrust bearing that promotes formation of a lubricating oil film and increases the load bearing capability of a thrust bearing. This taper-land type thrust bearing is constituted by an inclined surface having a deepest portion provided continuously with a groove, and a horizontal surface provided continuously with a rising gradient termination end of the inclined surface. A boundary line between the inclined surface and the horizontal surface inclines with respect to a radial direction. That is, the outflow direction of the lubricating oil is the direction of a resultant force of a rotating speed vector of the thrust collars rotating with the rotary shaft, and a velocity vector of a centrifugal force generated in the lubricating oil by the rotation. In PTL 2, the boundary line inclines so that a radial outer region is directed to a direction reverse to a rotational direction, to suppress outflow of the lubricating oil.
A thrust bearing device for a supercharger as intermediate technology (not well-known) conceived before the present inventors has reached the invention is shown in FIGS. 8 to 10. In the drawings, in a thrust bearing device 100, a turbine-side thrust collar 106 is made to extend in an axis-orthogonal direction (radial direction), and is locked to a step 104 formed at the rotary shaft 102. A compressor-side thrust collar 108 is made to confront the turbine-side thrust collar 106, and is fixed to the rotary shaft 102 with a hold-down member 110 fitted to the rotary shaft 102. A thrust bearing 112 made to extend in the axis-orthogonal direction (radial direction) with a larger diameter than the thrust collars 106 and 108 is interposed between the two thrust collars 106 and 108. The two thrust collars 106 and 108 rotate in the direction of arrow a together with the rotary shaft 102, and the thrust bearing 112 is fixed to a bearing housing (not shown). A thrust force applied to the rotary shaft 102 is received by the thrust bearing 112 via the two thrust collars 106 and 108, and thereby stops the movement of the rotary shaft 102 in a thrust direction.
A thrust bearing 112 is provided with an oil passage 112a that supplies lubricating oil r to sliding-contact surfaces that come into sliding contact with the two thrust collars 106 and 108. As shown in FIG. 10, the thrust bearing 112 is formed with a taper surface 114 including an inclined plane forming a rising gradient toward a downstream side in a rotational direction a of the rotary shaft 102, and a land surface 116 that is provided continuously with a rising gradient termination end of the taper surface 114 and forms the same plane as an outer peripheral plane of the thrust bearing 112. As shown in FIG. 9, a step 118 and a boundary line 120 that are formed between the taper surface 114 and the land surface 116 are directed to the radial direction. A centrifugal force b acts outward in the radial direction due to the rotation of the rotary shaft 12.
In this configuration, if a gap c in the axis-orthogonal direction (radial direction) on an outer peripheral side between the thrust collar 106 or 108 and the thrust bearing 112 is increased due to a manufacturing error or the like, a larger amount of lubricating oil flows out from the gap c when a large centrifugal force acts on the lubricating oil r. Accordingly, there is a concern that an oil film pressure between the thrust collar and the thrust bearing may deteriorate, and the load capability of the thrust bearing may decline extremely.